Method of manufacturing large capacity preforms by MCVD

Abstract

The invention provides a low cost method of manufacturing high capacity preforms by chemical vapor deposition. More particularly, there is described a method of manufacturing an optical fiber preform, which method comprises the steps of providing a substrate tube of silica doped with sufficient chlorine to obtain an OH concentration of less than 100 ppb and doped with sufficient fluorine proportional to the chlorine doping to obtain a refractive index that is lower than that of a natural silica, depositing inner cladding and an optical core inside the substrate tube, collapsing the substrate tube to form a primary preform, and depositing outer cladding of said natural silica on the resulting primary preform. The invention is applicable to manufacturing optical fibers.

Description

[0001] The invention relates to a method of manufacturing large capacity optical fiber preforms using modified chemical vapor deposition (MCVD) methods.[0002] The optical fiber is obtained by proportional hot drawing (fiber drawing), i.e. drawing while maintaining the various different portions of the preform.[0003] Preforms can be obtained by CVD methods such as MCVD or by the vapor axial deposition (VAD) method.[0004] When depositing by MCVD, successive layers of oxide compounds are deposited inside a silica tube referred to as a substrate tube, and they are then vitrified by means of a torch. These layers correspond to the core and to the inner portion of the cladding of the preform. They present a refractive index that varies in compliance with the properties that are desired for the optical fiber.[0005] It is desirable for reasons of keeping down method costs to obtain preforms that are of large capacity. It is also desirable to have optical fibers that are of great length so a...

AI Technical Summary

Benefits of technology

[0017] It is desirable to have a method that is less expensive and that makes it possible to obtain large capacity preforms using the MCVD method.

[0025] Chlorine doping serves to reduce the presence of OH groups which are responsible for undesirable attenuation of transmission in the optical fiber. However, this doping modifies the refractive index in two ways: firstly by means of a "chemical" effect, and secondly by means of a "physical" effect. The term "chemical" effect means that the index is increased by the very presence of chlorine. The term "physical" effect means that the index is increased due to the appearance of local stresses during fiber drawing. These stresses are associated with a reduction in the softening temperature because of the presence of the chlorine. The viscosity of the substrate tube is lower than that of pure silica.

[0026] The use of natural silica for building out serves to reduce the cost of manufacturing preforms considerably. The term "natural" silica is used to mean silica which is not obtained by synthesis, e.g. by decomposing silicon halides. Natural silica is also referred to as "quartz", where reference to "alpha" quartz means the most widespread crystal phase of silica. Natural silica is low in cost but its composition is also slightly different from that of the synthetic silica used for making substrate tubes. As a result the refractive index, the viscosity, and the expansion coefficient of various natural silicas differ from those of the substrate tubes that are presently available.

[0027] Thus, a preform obtained using a conventional substrate tube built out by means of natural silica will generally present a refractive index step between the substrate tube and the outer cladding. This index step in the preform is transmitted to the fiber and can indeed be amplified, and in the fiber it leads to optical effects that are undesirable. These effects include, for example, the effect whereby such fibers are observed to be more sensitive to losses due to microbends. The length of the cutoff wavelength can also be affected.

[0030] The substrate tubes used present a concentration of OH groups of less than 100 ppb. They thus make it possible to manufacture preforms having a small ratio b:a. This ratio is preferably less than 2.5.

Problems solved by technology

Unfortunately, when the core is close to the substrate tube, purity requirements for the substrate tube material becomes increasingly strict.

However this layer can be very thin if a high purity substrate tube is used.

The deposition rate of MCVD is limited by the transfer of heat through the substrate tube.

That technique suffers from the drawback of the very high cost of synthetic silica jackets.

Images